The study explored the effect after six hours of daylight exposure, or similar light via a specific lamp, against the effect after six hours by traditional office lighting (not daylight). It was shown that subjects working by traditional office lighting became tired and slow, whereas the mind woke up in more than one sense after a day spent by daylight. The effect lasted into the evening.
See the study here
Effects of prior light exposure on early evening performance, subjective sleepiness, and hormonal secretion
Münch M, Linhart F, Borisuit A, Jaeggi SM, Scartezzini JL.
Behavioral Neuroscience, 2012 Feb;126(1):196-203.